Molded articles made of expanded polystyrene foamed articles are widely employed, especially as heat insulating materials, packaging materials or cushioning materials, but have a disadvantage of low heat resistance, e.g., about 70.degree. to 80.degree. C. at the highest.
The problem of low heat resistance can be solved by using polypropylene foamed articles or styrene-acrylonitrile-butadiene copolymer (ABS resin) foamed articles.
Expanded polypropylene particles can be prepared by a process comprising dispersing polypropylene particles in water in a closed vessel, feeding a blowing agent to the vessel, heating the dispersion to a temperature of from the softening point of the polypropylene particles up to a temperature higher than the melting point of the polypropylene particles by 20.degree. C. while keeping the inner pressure at the vapor pressure of the blowing agent or higher, and opening an outlet provided below a liquid level of the closed vessel to thereby release an aqueous dispersion containing polypropylene particles impregnated with the blowing agent into an atmosphere having a pressure lower than the inner pressure of the closed vessel. For details, reference can be made to it, e.g., in Japanese Laid-Open Patent Application Nos. 12035/82, 25336/82, 90027/82, 195131/82, 1732/83, 23834/83, 25334/83, 33435/83, 55231/83, 76229/83, 76231/83 to 76234/83 and 87027/83. According to this process, expanded polypropylene particles having a bulk density of from 0.026 to 0.06 g/cm.sup.3 can be obtained. It is known that the above process is also applicable to production of polyethylene particles or crosslinked polyethylene particles as described in the above-cited publications.
The expanded polypropylene particles can be molded into a core of bumper or a container by incorporating air or nitrogen gas in the particles to impart secondary expandability, charging the expandable particles in a cavity of mold having steam vents, and heating the particles with steam of from 1.5 to 6 kg/cm.sup.2 G in pressure to expand and fuse with each other, followed by cooling. The molded articles of expanded polypropylene particles can also be obtained by compressing the expanded polypropylene particles with pressurized nitrogen gas or air, charging the compressed particles into a cavity of mold, and heating the particles to fuse with each other, followed by cooling.
The above-mentioned process for producing highly expanded polypropylene particles cannot be applied to production of expanded ABS resin particles because a non-crystalline ABS resin having high heat resistance and high bending strength behaves differently from crystalline polypropylene.
In the production of expanded polypropylene particles, the temperature for heating the aqueous dispersion of polypropylene particles is usually controlled between a range of from a temperature lower than the melting point of polypropylene by 10.degree. C. to a temperature higher than the melting point of polypropylene by 5.degree. C. in order to prevent fusion among polypropylene particles. It is considered in the art that impregnation of a volatile blowing agent into the resin particles is effected at non-crystalline portions or voids formed by shrinkage accompanying crystallization but not at crystalline portions. It appears that the polypropylene particles in the aqueous dispersion are plasticized by the presence of the volatile blowing agent to have a decreased apparent melting point.
Therefore, it is anticipated that a volatile blowing agent must be sufficiently impregnated into the non-crystalline ABS resin particles at around a glass transition point (Tg) that corresponds to a melting point of crystalline resins. Nevertheless, the volatile blowing agent can hardly be impregnated into center of the ABS resin particles in an aqueous dispersion system for some unknown reasons, failing to obtain expanded particles.